OK, let's work through this together. We're starting with 312 and we're going to multiply it by 91, which means we add 312 to itself 91 times.
Step 1: 0 + 312 = 312
Step 2: 312 + 312 = 624
Step 3: 624 + 312 = 936
Step 4: 936 + 312 = 1248
Step 5: 1248 + 312 = 1560
Step 6: 1560 + 312 = 1872
Step 7: 1872 + 312 = 2184
Step 8: 2184 + 312 = 2496
Step 9: 2496 + 312 = 2808
Step 10: 2808 + 312 = 3120
Step 11: 3120 + 312 = 3432
Step 12: 3432 + 312 = 3744
Step 13: 3744 + 312 = 4056
Step 14: 4056 + 312 = 4368
Step 15: 4368 + 312 = 4680
Step 16: 4680 + 312 = 4992
Step 17: 4992 + 312 = 5304
Step 18: 5304 + 312 = 5616
Step 19: 5616 + 312 = 5928
Step 20: 5928 + 312 = 6240
Step 21: 6240 + 312 = 6552
Step 22: 6552 + 312 = 6864
Step 23: 6864 + 312 = 7176
Step 24: 7176 + 312 = 7488
Step 25: 7488 + 312 = 7800
Step 26: 7800 + 312 = 8112
Step 27: 8112 + 312 = 8424
Step 28: 8424 + 312 = 8736
Step 29: 8736 + 312 = 9048
Step 30: 9048 + 312 = 9360
Step 31: 9360 + 312 = 9672
Step 32: 9672 + 312 = 9984
Step 33: 9984 + 312 = 10296
Step 34: 10296 + 312 = 10608
Step 35: 10608 + 312 = 10920
Step 36: 10920 + 312 = 11232
Step 37: 11232 + 312 = 11544
Step 38: 11544 + 312 = 11856
Step 39: 11856 + 312 = 12168
Step 40: 12168 + 312 = 12480
Step 41: 12480 + 312 = 12792
Step 42: 12792 + 312 = 13104
Step 43: 13104 + 312 = 13416
Step 44: 13416 + 312 = 13728
Step 45: 13728 + 312 = 14040
Step 46: 14040 + 312 = 14352
Step 47: 14352 + 312 = 14664
Step 48: 14664 + 312 = 14976
Step 49: 14976 + 312 = 15288
Step 50: 15288 + 312 = 15600
Step 51: 15600 + 312 = 15912
Step 52: 15912 + 312 = 16224
Step 53: 16224 + 312 = 16536
Step 54: 16536 + 312 = 16848
Step 55: 16848 + 312 = 17160
Step 56: 17160 + 312 = 17472
Step 57: 17472 + 312 = 17784
Step 58: 17784 + 312 = 18096
Step 59: 18096 + 312 = 18408
Step 60: 18408 + 312 = 18720
Step 61: 18720 + 312 = 19032
Step 62: 19032 + 312 = 19344
Step 63: 19344 + 312 = 19656
Step 64: 19656 + 312 = 19968
Step 65: 19968 + 312 = 20280
Step 66: 20280 + 312 = 20592
Step 67: 20592 + 312 = 20904
Step 68: 20904 + 312 = 21216
Step 69: 21216 + 312 = 21528
Step 70: 21528 + 312 = 21840
Step 71: 21840 + 312 = 22152
Step 72: 22152 + 312 = 22464
Step 73: 22464 + 312 = 22776
Step 74: 22776 + 312 = 23088
Step 75: 23088 + 312 = 23400
Step 76: 23400 + 312 = 23712
Step 77: 23712 + 312 = 24024
Step 78: 24024 + 312 = 24336
Step 79: 24336 + 312 = 24648
Step 80: 24648 + 312 = 24960
Step 81: 24960 + 312 = 25272
Step 82: 25272 + 312 = 25584
Step 83: 25584 + 312 = 25896
Step 84: 25896 + 312 = 26208
Step 85: 26208 + 312 = 26520
Step 86: 26520 + 312 = 26832
Step 87: 26832 + 312 = 27144
Step 88: 27144 + 312 = 27456
Step 89: 27456 + 312 = 27768
Step 90: 27768 + 312 = 28080
Step 91: 28080 + 312 = 28392

So, 312*91 = 28392